In reply to brunchman:
What they do is overpower the smell from the climbing shoes with a stronger one from aromatic oils. My wife insisted I chucked them out because she found the smell from the oil worse than that from the shoes (!). Admittedly we were on a trip so the shoes and Boot Bananas were in the same room we slept in, not like at home where the shoes would be in a cupboard.
I don't buy the moisture absorbtion claims because the UK gets humid and the dessicant is going to absorb moisture from the air until it becomes ineffective.
After trying the Boot Bananas, various sprays, a UV machine at a wall in Austria, freezing the shoes and washing them I found what actually works is to start with a fresh pair of shoes, wear socks and get the shoes out the bag and somewhere they will dry out as quickly as possible.